The universe does not need either, if the darkness wins everything will kill each other until only one is left, if the light wins thing still be much more prosperous but death will still exist and there will still be war just not necessarily to the point of universal genocide. It was never the light's goal to make everything undying, the only ones that were made so are a relatively few of one specific species and that was a result of the darkness itself. In short the universe does not need the darkness for things to die, it does not need the light for things to live and if both simply left life will go on being created and destroyed, same if both remained. The world of light and world of darkness that the nine to on about was pulled from their dark matter encrusted asses, the same goes for Mara and Ulan Tan.